Okay, this is pretty much my first foray into tubes although I've done my share of DIY on other fronts. I've got an Aikido LV w/ 12BH7 and 48v .52A Jameco Meanwell wall-wart. I powered it up, nothin' happened, then I realized I forgot to bridge the heaters in series. Did that on the 'test bench' and things got a-glowin' okay. When I moved the Aikido over to my gear and powered up (admittedly with the switch on already), nothing. And I haven't been able to get anything to happen since.

The board is simple and I've looked over the solder job and wiring and PS, etc. I've used my DMM and get 48v exact reading from each tube, so something is happening. The tubes are rated for 300mA or so current so my wall-wart should be okay, but I am totally stumped. Any ideas? I got no glow, no nothin' after that one time.

Heaters are nearly short circuit when cold! resistance increases as they warm up so:

You need to give more information. However if you powered the heaters in parallel then they would warm slightly, the resistance would increase then you connected in series the heaters would start O.K.

You then let them cool down and the heater resistance would drop to nearly short circuit so they won't start again. It may be you need a soft start. Try putting a 10 Ohm resistor in series for about 30 seconds then short it out and see if they run up!

You need to measure the voltage from your wall wart when the tube heaters are powered. It may be in current limit. IE. very low volts due to heater "short" due to heaters being cold!

What Voltage are you running your heaters at?
Are the heaters OK not open circuit?

Heaters are nearly short circuit when cold! resistance increases as they warm up so:

You need to give more information. However if you powered the heaters in parallel then they would warm slightly, the resistance would increase then you connected in series the heaters would start O.K.

You then let them cool down and the heater resistance would drop to nearly short circuit so they won't start again. It may be you need a soft start. Try putting a 10 Ohm resistor in series for about 30 seconds then short it out and see if they run up!

You need to measure the voltage from your wall wart when the tube heaters are powered. It may be in current limit. IE. very low volts due to heater "short" due to heaters being cold!

What Voltage are you running your heaters at?
Are the heaters OK not open circuit?

RegardsM. Gregg

Hi Gregg, what I am doing so far is checking heater (pin 5) voltage by placing one probe on the (+) of the PS input and the other on the heater pin. I get 48v when I do this. Please let me know if this is the wrong way to check.

Where exactly do I place the 10 ohm in series? With the (+) input voltage?

Ok, so you're saying there is a strong current draw which may exceed the wall-wart when turning on, so the resistor 'soft start' would help with this?

Hi,

How are you producing the HT "Inverter"?

The resistor is only in the circuit for 30 seconds it will get hot so you need a wirewound high wattage type 5W or more then short it out with a piece of wire so its not in circuit. If the heaters start then you have your answer!

OK. Tests are:

Power off check the resistance across each of the tube heaters.

If the resistance is O.K. then check the voltage across each heater when powered.(Take care if you have lifted the heaters with HT).

Test the voltage across the heater chain. IE. across the two in series.

Hi Gregg, what I am doing so far is checking heater (pin 5) voltage by placing one probe on the (+) of the PS input and the other on the heater pin. I get 48v when I do this. Please let me know if this is the wrong way to check.

Where exactly do I place the 10 ohm in series? With the (+) input voltage?

The 10 Ohm resistor is in series with the tube heater chain. IE. The + wire to the first tube heater connection. I would turn off the HT until you have solved this problem.

12bh7 heaters are 12.6 volt so two in series would be 25.2 Volt. So how are you reducing the 48V?

The 10 Ohm resistor is in series with the tube heater chain. IE. The + wire to the first tube heater connection. I would turn off the HT until you have solved this problem.

12bh7 heaters are 12.6 volt so two in series would be 25.2 Volt. So how are you reducing the 48V?

Regards
M. Gregg

Here's a pic of the the circuit. This is a PCB going straight to a 48v wall-wart smps (linked in the first post), so I'm not sure if turning off the HT separately is do-able. It's late so I'll have a go later in the morning.

You don't need the 10 Ohm. R17 does the same job! If you check from ground to junction of R17 and V4 heater then at junction of C1 or link you should have approx half the voltage of supply. If you have full supply then V1 or V3 is open. If this is OK the check between V1 & V3 heater. The current drawn should give a voltage drop across each of the valve heaters.